Water, e.g. from a rainwater-collection system, or sewage is typically held in an underground tank made of plastic. Such a tank is typically made by blow- or spin-molding from polyethylene.
In practice, underground plastic tanks are known in various configurations. These known underground plastic tanks frequently have a cylindrical shape for strength reasons. They must be strong enough to withstand the pressure of the soil on top of and around them. So as to ensure that they can withstand such soil pressure sufficiently, the known underground-plastic tanks often have thick walls.
The underground plastic tanks usually hold volumes of about 3500 liters to about 6000 liters and the most common volume capacities range between 5000 liters and 6000 liters. There are, however, also applications where significantly higher volume capacities are required. The diameter of known cylindrical underground plastic tanks usually ranges up to around 2 m. So as to achieve high stability and strength to resist soil pressure, particularly in the case of high-volume vessels, double-walled tank configurations are known. However, the manufacture of these underground plastic tanks is time-consuming, requires a considerable amount of material and is expensive, and in addition these vessels have the disadvantage of being very heavy.
When cylindrical underground plastic tanks with small cylinder diameters are used, the wall thickness can also be reduced. From practice, for example, an underground plastic tank with a relatively small cylindrical diameter is known, which is shaped as a tube ring. This vessel, however, holds only a relatively small volume. These underground tanks are not suited for larger volumes since the strength and resistance to outside pressure decreases with increasing size and the resultant increasingly large center opening. From practice, furthermore plastic underground tanks are known made up of several small vessels that are connected together to make up the required volume. The production of multipart tanks of this type requires a lot of material and is time-consuming.